In 1972, Phoenix sealed Sky Harbor's main runway with tire-rubber beside an untreated taxiway; 13 years later, untreated asphalt hardened 20 times more and cracked

Phoenix treated the main runway at Sky Harbor International Airport with an asphalt-rubber seal in June 1972 and left the taxiway adjacent to it untreated. The asphalt beneath the untreated taxiway had hardened more than 20 times as much as the asphalt protected by the rubber seal, which was the result of thirteen years of pavement sampling. Russell Howard Schnormeier, who was an engineer with the City of Phoenix, documented the comparison in a 1986 paper that was published in Transportation Research Record, with the two surfaces using otherwise identical asphaltic concrete. The main runway received a stress-absorbing membrane, or SAM, that was made from asphalt-rubber and small stone chips, while the adjacent taxiway did not receive the treatment.Researchers collected pavement cores from the runway and taxiway at intervals between 1972 and 1985, and then measured the viscosity of the asphalt cement that had been recovered. Viscosity is a measure of a material's resistance to flow, and changes in asphalt viscosity can help indicate how much the binder has hardened with age.The rubber seal limited asphalt hardeningThe 1985 measurements showed a substantial difference between the two surfaces, with one comparison showing that asphalt from the treated runway had a viscosity of 1,125 poises at 140°F, while the untreated taxiway's asphalt measured 92,500 poises; another comparison recorded more than 200,000 poises in the untreated section, compared with 1,411 poises in the treated section. Schnormeier reported that the untreated asphalt had undergone more than 20 times as much hardening from oxidation and weathering, and the asphalt beneath the rubber seal showed no increase in viscosity that could be appreciated over the 13-year period.The difference was attributed to the seal's ability to limit the loss and reaction of volatile components in the asphalt, with the untreated material becoming brittle and cracked, while the treated surface was able to retain more of its flexibility. The paper also reported that crack reflection was prevented and surface maintenance had been eliminated for 13 years, although the runway required periodic high-pressure water cleaning to remove rubber buildup from aircraft tires. The test measured the properties of asphalt recovered from pavement cores, and not just the visible condition of the runway, with the viscosity measurements providing a numerical comparison of how the binder aged under the two treatments.How recycled tire rubber works in pavementAsphalt-rubber is made by blending asphalt cement with ground tire rubber, and the Federal Highway Administration explains that crumb rubber can modify the binder's properties, including its elasticity and resistance to aging. A stress-absorbing membrane is formed by applying hot asphalt-rubber and covering it with aggregate chips, and when it is used on an existing pavement, the layer can seal the surface and reduce the reflection of smaller cracks through the treatment. Its performance depends on the condition of the underlying structure and the way the treatment is installed, and does not eliminate every type of pavement failure.A 1991 study by Equbalali Charania, Joe A. Cano, and Schnormeier reviewed the city's wider experience with the material, which included its use on streets and at Sky Harbor, and described the treatment's ability to retard cracking and preserve asphalt properties, while noting that the performance varied with pavement conditions and construction details. The airport comparison is still a very specific example of the treatment's reported effect on asphalt aging, and after 13 years, the treated runway's asphalt retained far more of its original viscosity than the untreated taxiway's asphalt.You use AI every day. Now get your AI Quotient. Take the AIQ test.